The present invention relates to a method for producing printed circuit boards, in particular multilayer circuit boards, using both substractive and additive methods.
The printed circuit board structure according to the invention is based on both subtractive and additive fabrication methods. In a subtractive method, a desired conductor pattern is etched from a copper cladding of a base material or substrate. In an additive method the conductor pattern is deposited on a dielectric base material. Attainment of reliable adhesion has been a problem in the additive methods, as has been the substandard thermal stability of acrylate-butadiene insulating materials used in the method. Newer compositions of photoimageable polymer materials of higher stability, which are under development, facilitate the production of printed-circuit boards of higher quality and precision using additive methods. These advances in additive printed circuit board (PCB) techniques can also be employed in the production of high-precision multilayer boards. The cost-efficiency of this production technology surpasses by a wide margin the conventional lamination method for producing multilayer circuit boards.
With the decreasing size of drilled holes, increasing demands on the conductor density of printed circuit boards raises drilling costs significantly. Fabrication of buried interconnections using conventional production techniques of multilayer boards also requires drilling prior to lamination. By contrast, use of a photoimageable polymer intermediate insulation makes it possible to fabricate the holes of buried interlayer vias while at the same time photoimaging the polymer insulating layers. Along with the increase in packaging density, the base materials become thinner. Thinner materials facilitate the fabrication of vias by punching. Compared with drilling, punching is an appreciably cheaper production method.
In multilayer circuit boards, the interconnections between laminated layers are made by metallization of the drilled holes. To produce a reliable contact, the drilled hole must have a smooth bore surface and the bore surface profile in the copper foil around the drilled hole must also be smooth.